It’s always been a fantasy of ours, with some mods trying to do it, but failing horribly.

In 2007, Kim Swift teased players with the possibility of Portal multiplayer, by saying:

“We’re still playing it by ear at this point, figuring out if we want to do multi-player next, or Portal 2, or release map packs.”

When Kim confirmed that Portal 2 is in development on February 21st 2008, in an interview with Shacknews, and when it was finally formally announced on March 5th 2010, most thought this was the end for Portal multiplayer, but, in an interview with 1UP, Erik Wolpaw says they actually tried to do competitive multiplayer in Portal 2.

“Along with co-op, [we had] the idea of sort of a competitive Portal multiplayer, we went down that path, actually, for a little while and had something up and running — the best way to describe it is sort of speedball meets Portal. You know, a sports analog. And it quickly became apparent that while it’s fun for about two seconds to drop portals under people and things like that, it quickly just devolves into pure chaos. It lost a lot of the stuff that was really entertaining about Portal, which was puzzle-solving. Cooperative puzzle-solving was just a much more rewarding path.”

Apparently, this competitive MP component would have involved moving a ball from one end of a space to the other quickly using portals.

The competition would try to stop the player by, for example, trapping them with a portal shot onto the ground, but like Erik said, it would soon devolve into inane chaos.

That wall with all the PC magazines with Valve game articles in them gave me a lot more faith in Valve, Releasing games for the Xbox then Mac and PS3 started me get concerned if Valve goes the same path Rockstar Games did (starts career on PC, then continues only on consoles).
The amount of PC magazines on this wall compared to the one or two Xbox magazines there are shows how much Valve is centralized on the PC, and hopefully will stay so.